1. Field of the Invention
The present invention relates to improved methods of cementing casing strings in well bores, and more particularly, to methods of cementing conductor or surface casing strings in deep water offshore wells.
2. Description of the Prior Art
In carrying out completion operations in oil and gas wells, hydraulic cement compositions are commonly utilized. For example, hydraulic cement compositions are used in primary cementing operations whereby casing strings are cemented in well bores. That is, a hydraulic cement composition is pumped into the annular space between the walls of a well bore and the exterior of a casing string disposed therein. The cement composition is permitted to set in the annular space thereby forming an annular sheath of hardened impermeable cement therein. The objective of the cement sheath is to physically support and position the casing string in the well bore and bond the casing string to the walls of the well bore whereby the undesirable migration of fluids between zones or formations penetrated by the well bore is prevented.
Primary cementing operations in deep water offshore wells are particularly difficult in that they are carried out in well bores which penetrate formations between the sea floor or mud line and a depth generally under about 2,000 feet below the mud line. Such formations are often not well consolidated, readily fracture and often have highly pressured water flows therethrough. For example, the fracture gradients in subsea well bores in which conductor and/or surface casing strings are cemented limit the densities of the cement compositions utilized to 11 or 12 pounds per gallon. The high water flows through the well bores often wash away cement compositions which have densities below 10 to 11 pounds per gallon.
Another problem involved in deep water offshore well cementing is the temperature at which the cement composition must set. Deep water offshore wells typically have sea bottom temperatures ranging from about 32.degree. F. to 55.degree. F. depending on the geographical location. The cement compositions utilized for performing cementing operations at such temperatures must set and provide enough compressive strength to proceed with drilling without involving long waiting-on-cement (WOC) times, preferably less than 24 hours. Accordingly, the cement compositions must include set and strength accelerating agents to allow the cement compositions to set at the low temperatures involved and develop early compressive strengths. However, a problem in the use of set and strength accelerating agents is that they often cause the cement compositions to have thickening times which are too short to allow placement of the cement compositions in the formations or zones to be cemented. Thus, the cement compositions used in deep offshore wells must have relatively long thickening times to allow them to be pumped to the location to be cemented, but at the same time they must set and develop sufficient compressive strengths to allow further drilling as quickly as possible. The generally accepted requirements for cement compositions to overcome the above described problems in the Gulf Coast region of the United States include cement composition densities in the range of from 10 to 12 pounds per gallon, thickening times of from 3 to 5 hours and compressive strengths of from 400 to 600 psi at temperatures of from about 45.degree. F. to about 55.degree. F.
While methods and cement compositions have been developed and used heretofore for cementing in deep water offshore wells, there is a continuing need for improved methods and cement compositions whereby the cement compositions have pumping times which are sufficient to allow their placement in the zone to be cemented, but after placement they quickly set and develop high compressive strengths at low temperatures.